The invention relates to a position sensor, specifically an angle of rotation sensor with a CCD line.
CCD sensors are known, such as is described in the application brief by the Texas Instruments under the title xe2x80x9clinear products TLS 214 integrated opto-sensorxe2x80x9d in connection with the FIGS. 1 through 5 shown there. Details of the integrated TLS 215 opto-sensor were published in section 4.19 through 4.28 of the Texas Instruments product description and under the heading xe2x80x9cTSZ 215, 128x1 integrated opto-sensorxe2x80x9d under number SOES 005Axe2x80x94May 1993.
The known CCD line has a row of composite elements called pixels arranged side by side. Starting from an initial state in which the pixels are drained of all electrical charge, these composite elements are then charged in proportion with the light flux striking them. The varying charge on the pixels resulting from the difference in light distribution striking them corresponds to different voltage values on the pixels. At a predetermined, periodically repeated point in time, the voltages of all pixels are transmitted in parallel to the respective memory elements of a shift register, so that the voltage distribution on the individual memory elements in the shift register corresponds to the light flux supplied to the respective individual pixels. By means of suitable electronics, the composite elements (pixels) are then reset at their initial charge free state and again charged by the incident light, while at the same time the voltage values are read incrementally in series out of the memory elements of the shift register, and the analog values are converted to digital values. Thus, for example, 64 to 128 digital values are ultimately available, describing the light flux striking the respective pixels during the past time interval. With the help of a carrier with coded apertures where the light distribution is allowed to pass through corresponds to the position of the carrier with respect to the CCD line, it is thus possible to draw conclusions about the instantaneous position of the carrier during the measurement process on the basis of the measured light and voltage distribution. In the example described here, this applies to the angle of rotation of a coded disk, but also to the description of a longitudinal position of a carrier, when the linear position of the carrier is described with the help of an optically light-transmitting code. The use of such optically light-transmitting code has already been described in German Patent 195 32 903.1. The essential difference in comparison with the code disk described there is that the light flux penetrating the disk is measured not with a CCD line, but with one or more diodes. The invention is therefore based on a position sensor, in particular an angle of rotation sensor of the generic type in the definition of the species in claim 1. One disadvantage of such a sensor device is that during a sampling interval of the light flux distribution of the light flux passing through the code disk, the code disk undergoes a considerable shift with a rapid turn of the steering wheel. In other words, there is a change in the allocation of individual light-transmitting fields on the code disk with respect to the pixel, so that the light flux distribution of the incident light changes during a collecting period. This results in blurred grey scale value transitions, so that the image of the light distribution due to the code disk on the CCD line is blurred. To reduce the blurred grey scale value transitions, one might attempt to keep the number of code words and hence the code as large and as rough as possible. A rough code also serves to reduce sensitivity to dirt and to simplify code disk production. The object of the present invention is to greatly improve the known system by improving the grey scale value transitions through other measures and increasing the sensitivity of the sensor.
The invention consists in principle of greatly shortening the lighting duration of the code element, i.e., of the rotation sensor of the code disk. The code element is lighted only briefly, approximately in the manner of a stroboscope. One disadvantage of the invention could be that the light flux output and thus the charge picked up by the collecting elements is comparatively low. This situation can be remedied where the light source is operated so that the rated output is obtained for the entire period. Thus, in other words, a large amount of light is delivered by the light source for short periods of time. To be able to determine unambiguously the position of the code element after the charging process on the basis of the reduced light flux, the on-time of the light source is preferably synchronized with the sampling time and the readout timing. Tests have shown that the cyclic on-time corresponding to the features according to claim 3 should amount to less than 5% of the cycle time and, better yet, less than 1% of the cycle time.
To further improve the measurement accuracy of the sensor of the generic type the code element (i.e., the code disk for an angle of rotation sensor) is provided with code fields, which correspond to only two states. In other words, the code element is provided with code fields that either are opaque or have a defined light transmission. The light transmission corresponds to states 0 and 1 of a multistep code, where the geometric arrangement is such that increasing numbers of memory elements (pixels) are either covered by a code field or receive the light flux. It would also be conceivable to assign a separate code element to each pixel. The proposed invention serves the goal of greater security, in order to be able to decide in borderline cases whether or not a sufficient light flux has been sent to a group of pixels. The decision regarding whether or not it was a light-transmitting code field or an opaque code field is made much more reliably than when only a single pixel is present per code field. Furthermore, multiple code fields (for example, 12 code fields) are scanned by the CCD line and weighted by the sensor, with several pixels again being assigned to each code field, e.g., between 10 and 11 pixels (this is obtained, for example, by using a 12-step maximum code and a code line with 128 pixels). Using a larger number of pixels per code element or coding step of a multistep code has the advantageous effect that a light suppressing code field or a light flux transmitting code field can be recognized even when the pixel voltage, which determines the flank of the image, assumes an intermediate value. This is the case, for example, when the edge of a code field only partially covers a pixel or when stray radiation at the edges of the code field distorts the flanks readable on the pixels.
When the position sensor for determining the angle of rotation according to the invention is used in particular to determine the angle of rotation of a steering wheel, it is preferable to form the code fields of the code disk by adjacent sectors of a circle with the same sector angle In principle, the code element may be formed from a code disk on which light-transmitting and opaque sectors of a circle are arranged adjacent to one another like pieces of a pie in a sequence determined by the code. The code fields may follow one another on an ring, as if only sections inside the ring were cut from the aforementioned pieces of pie. In this case it is advisable to have the CCD line run at a tangent to the ring and above or below the code disk over the ring. Since the line is essentially a straight line, while the circular sector is curved, the light flux allowed to pass through by the light transmitting code fields also depends on the position of the respective pixels on the line. Suitable corrections can be made here accordingly to compensate for the regular reduction in light flux. In a further development of the invention, a maximum code is selected. This maximum code is characterized in that the code elements arranged in an arc in an ordered sequence of code elements lead to non-repeating coded values. In other words, 180 code fields can be accommodated on an ring, for example, where 12 successive code elements always yield a 12-step code word. If the circular disk is shifted by an angle of 2xc2x0, i.e., by one code element, the result is a new code word. All 180 possible code words are different from one another, and a new first step is always added and the last step is omitted only when there are adjacent code fields (or reversed with the opposite direction of rotation).
To achieve a greater redundancy, the combination begins with a maximum code of 4,096 code words, using only a comparatively small section of this code word sequence which includes 180 code words. One has essentially free choice as to which section of 180 code words is to be selected from the total sequence of 4,096 code words and applied to the code disk in an annular form. It is advisable for the CCD line to be arranged so it is tangential to an ring for a specific embodiment of the code field.
Another possible arrangement of the code line is given where the code line is arranged radially. Therefore, the steps of the multistep code word read out of the code line must be arranged radially side by side. Such a radial arrangement may be expedient, for example, when the extent of the available ring is comparatively small, as with a lateral surface of a steering column assembly. However, the code words arranged radially need not be arranged on the lateral surface area of a hollow cylinder, they may also run radially on a circular disk. In both cases, a Gray code is preferably used.
To be able to use microprocessors in analyzing the code words read out, it is advisable in a further refinement of the invention to use the combination of features according to claim 11.
According to this, the code words analyzed are analyzed by a microprocessor, by checking for errors, and determining the measured angle on the basis of the code word thus found.
It was explained above that a step in a multistep code is not determined by a single pixel, but instead the curve contour of this step is practically circumscribed by the voltages of several pixels.
Now, while the corresponding (12-step) code word can be found with a knowledge of the curve determined by the 128 pixels, and thus the angle value can be determined to 2xc2x0 with sufficient approximation, various schemes are set forth to determine the angle with a greater accuracy. Thus, not only is the multistep code word determined in general, but also its position relative to the beginning and end of the CCD line is determined. This yields a great increase in accuracy in determination of the precisely measured angle. In other words, if one finds that the edge of one or more steps is shifted to the right or left, e.g,. by 1 pixel relative to the normal position, then the measured angular position can be determined with a 10-fold accuracy in this way, since approximately 10 pixels describe a coding step, which results due to the code field. In this way, one can use various strategies. For example, the shift in the flank or the shift in the maximum of one or more coding steps relative to a normal position can be determined.
With corresponding analysis algorithms (software), a resolution in the sub-pixel range can be achieved. To increase accuracy, the contrast between reproduction of light-transmitting code fields versus opaque code fields can also be determined. A reduction in differential values (contrast) be detected here due to soiling of code fields due to some dirt on the light-transmitting fields. If the contrast sinks below a certain threshold, it may mean that the disk needs to be cleaned or replaced. The transmitting power of the light source be adjusted to restore the desired contrast by increasing the transmitting power or lengthening the transmitting time.
This principle can be used with all CCD line sensors and is an additional independent embodiment to improve the sensitivity of the sensor. In analysis of the analog values provided by the CCD line, the analog value is first converted to a digital number which describes the amplitude value of the pixel voltage and then the measured angle of rotation value is calculated on the basis of the digital number, with all the additional computations such as error correction and other test steps. Since the conversion of analog value into digital value can take place in parallel with calculation of the angle value, it is proposed to use on A-D converter and a computer in parallel. These measures can be used for all sensors. If a digital value is formed through converting an analog value, then conversion of the next value is triggered by an interrupt, and after that, calculation of the angle value by the computer is continued.
As mentioned above, the invention relates to an angle of rotation sensor, specifically a steering angle sensor for motor vehicles. Such sensors are required for the control systems for controlling the driving stability of motor vehicles. With such sensors, at least one circular code track is usually provided on a code element, with non-repeating code words applied to the code track. The code element is linked to the steering column assembly of the motor vehicle. In this way, the slewing motion of the vehicle""s wheels can be deduced from the rotational movement of the code element.
Since only one specific code word can be read by the reading device in each rotational position of the code element, the position of the code disk and hence the angular position of the steering wheel of the vehicle can be deduced on the basis of the code word found. One problem is that the steering wheel must make several revolutions to execute the total possible slewing motion of the steering wheel. Thus it is not only important to know the absolute rotational movement of the steering wheel, but also the number of revolutions of the steering column assembly with respect to a zero position must be known to be able to determine definitively the slew angle of the steering wheel. For this purpose, German Patent Application No. P 8636 by the present applicant has already proposed that a sequence processor be provided with a counter, which is incremented or decremented as a function of the revolutions executed by the code element, so that the number of the revolutions of the code element with respect to a zero position is always known.
The object of the invention is to eliminate the need for a sequence processor for determining the number of revolutions currently in effect and to integrate the measurement of revolutions into the actual angle of rotation sensor.
The invention consists in principle of providing several code tracks arranged across the direction of movement of the reading device and advancing the reading device to the individual code track according to the number of revolutions currently in effect. Thus the instantaneously valid revolution can be determined on the basis of the code track just scanned. This may take place by scanning within each track a special code which indicates the number of revolutions. However, it is also possible to distribute a continuous code (for example, a maximum code) over all the tracks in such a way that not only the absolute angle of rotation but also the number of rotations is determined at the same time with the help of the decoded code words.
An especially simple arrangement of code tracks results where the individual code tracks are connected in the form of a spiral. This may be accomplished, for example, by moving the reading device one track higher or lower, depending on its rotational movement, for a certain absolute angular position. However, the entire code track becomes especially simple when it forms a continuous spiral track, so that the direction of movement of the reading device does not undergo any great change, except for switching from forward motion to reverse motion. Then the code track can be arranged in a spiral form either on a circular disk or on the lateral surface of a cylinder. Both the transmitted light principle and the reflected light principle can be used. If the cylinder is not hollow, the reflection principle should be used, for example, when the code track is applied to the steering column assembly in spiral.
A large number of different conveyance means may be used to move the reading device. The conveyance device described in claim 20 has been found to be especially advantageous, in that the guidance means are arranged directly next to the code element and the reading device. It is especially effective when the guidance means assigned to the code element is run parallel to the spiral code track. The design becomes especially simple when the code on the code element works by the transmitted light principle. It is expedient here to arrange the code, i.e. the perforations on the code disk, on the bottom of the guide groove, because the disk is thinner there anyway. At the same time, the reading elements, such as diodes, CCD line, Hall element or the like may be arranged on the surface of the reading device facing the bottom of the groove, so that it is located directly beneath the passages forming the code in such a way as to be shielded on the sides.
To avoid having to establish a special code with regard to the number of revolutions at a given instant, it is preferred that the code is formed by a continuous maximum code that extends over the entire spiral. Since none of the code words are repeated, not only the absolute angle, but also the corresponding number of rotations can be encoded within these multistep code words also at the same time.
The reading device becomes especially simple when using a CCD line. However, the reading element, such as diodes or Hall elements, for example, can be pointed at tracks running parallel to one another, so that the absolute rotational position and also the relative rotational position of the code element can be determined with respect to the reading elements in this way.